Cal-P is an unofficial term for the railroad line from Martinez to Sacramento, California (or perhaps for the whole line from Oakland to Sacramento via Martinez). (Officially the line is the Martinez Subdivision from 10th St Oakland to Roseville.)
It hosts several Amtrak passenger routes, such as the Capitol Corridor, Coast Starlight, and California Zephyr. The name comes from the California Pacific Railroad (Cal-P), which built the line from Suisun-Fairfield to Sacramento (so some would say "Cal-P" only refers to that piece of railroad). The Cal-P was taken over by the Central Pacific, which in late 1879 completed the line from Suisun-Fairfield to Oakland via the train ferry from Benicia 38.045°N 122.166°W / 38.045; -122.166 to Port Costa 38.048°N 122.183°W / 38.048; -122.183. The present double-track lift bridge across the Carquinez Strait replaced the train ferry in 1930.
The California Pacific became part of the Southern Pacific Railroad (SP) which in the 1990s merged into the Union Pacific Railroad (UP), the line's current owner. UP operates numerous freight trains over the route.